Droid signal "bars" don't match -dBm strength?

[mobilecal]

I know bars aren't the greatest indicator of signal strength but I realized the Droid "bars" don't really match the signal strength of actual the actual -dBm reading.

Over the period of a few minutes:

-89dBm (3 bars)
-89dBm (2 bars)
-89dBM (0 bars)
-91dBM (2 bars)
-87dBm (0 bars)

This just doesn't make sense. At one point I had 0 bars at -89dBm yet showed 2 bars at -91dBm which is suppose to be weaker. Maybe I should just ignore the signal altogether.
NOTE: I had WiFi off.

 

[jjf]

I'm only making a wild guess here, but perhaps the "bar" measurement is taking a combination of RF signal strength and measured bit error rate. Do we know the bars strictly measure RF strength, or are they an indication of signal quality?